This invention relates to the conservation of the energy necessary to maintain the desired conditions in a building air conditioning system.
A typical commercial building air conditioning system designed to maintain a desired temperature setting and a desired humidity setting in a plurality of zones may comprise an outdoor air duct and damper to supply a desired volume of fresh air to the system, a return air duct and damper to control the amount of return air mixed with the outdoor air, a cooling coil to reduce the humidity of the air and/or cool the air to a desired temperature, a fan for supplying the air to the zones, a humidifier for humidifying the air supplied to the zones and a separate reheat coil associated with each zone to bring the air to the zones to a desired temperature.
Additionally, the air conditioning systems designed to control the temperature and humidity of buildings located in the northern climates may also have preheat coils to protect the cooling coils from freezing. These preheat coils are located between the source of outdoor air and the cooling coil to insure that the air supplied to the cooling coil is always at some selected temperature above the freezing point. Thus, even though the outdoor air temperature drops below freezing, the air supplied to the cooling coil will always be above freezing. In climates where the outdoor air temperature does not fall below freezing, the preheat coil may be eliminated.
In early air conditioning systems, the outdoor air damper was manually set to admit a predetermined amount of fresh air into the building. It was soon realized, however, that if the outdoor air damper was not periodically adjusted according to outdoor air temperature variations, the amount of energy required to maintain the preset indoor conditions varied widely. The outdoor air damper was, as a result, automated to vary the amount of fresh air admitted into the building in accordance with the outdoor air temperature as sensed by a thermostat.
Temperature alone, however, does not determine the total heat capacity of a given volume of air as does enthalpy. Enthalpy is determined from both temperature and humidity. Controls were, therefore, designed to modulate the outdoor air damper according to both outdoor air temperature and humidity. But no system has controlled this damper at a position determined by the temperature and moisture content of the outdoor air, return air and the air discharged to the zones; and, no system has controlled the damper at a position requiring the lowest total energy input as a function of the above parameters to maintain the desired conditions of the discharge air.